A double-convex lens, with each face having same radius of curvature R, is made of glass of refractive index n. Its power is : (A) $\frac{2(n - 1)}{R}$ (B) $\frac{(2n - 1)}{R}$ (C) $\frac{(n - 1)}{2R}$ (D) $\frac{(2n - 1)}{2R}$